1. Field of the Invention
The invention relates to the field of evaporative fluid conditioning. More specifically, the invention relates to the field of sensible cooling of fluids (gas, liquid or mixtures with and without phase changes) to substantially the dew point for gas by indirect evaporative cooling within a heat exchanger having canalized gas and fluid flows and a lateral temperature gradient across the heat exchange plates. Further the invention herein is directed to an improved membrane design, to aid in quick distribution of liquid, efficient reservoir system and a control system.
2. Discussion of the Background
Indirect evaporative cooling is a method of cooling a gas stream, usually air, by evaporating a cooling liquid, usually water, into a second air stream while transferring heat from the first air stream to the second. The method has certain inherent advantages compared to conventional air conditioning: low electricity requirements, relatively high reliability, and the ability to do away with the need for refrigerants such as R-134 and all the disadvantages they entail. However, indirect evaporative cooling is as yet only used in specially built commercial applications, and is not yet available on the market as a residential or after market product. This is due to certain disadvantages of known indirect evaporative coolers and methods: excessive cost, ineffective thermodynamic cycle that does not allow for enough cooling of air for the expenses involved, ineffective water supply system, scale buildup, poor or expensive heat exchanger, excessive pressure drop, difficulty approaching the dew point of the air stream (the theoretical limit of cooling), relatively high dew points in humid atmospheres, large pressure drops across the heat exchange apparatus, large heat exchange apparatus, and in some designs, reliance on a good deal of ancillary equipment.
U.S. Pat. No. 4,002,040 issued to Munters et al on Jan. 11, 1977 discloses a heat exchanger in which there is no mixing between canalized air streams, and in which the air stream passing through the device is put through a 270 degree turn within the device, resulting in a large pressure drop engendered by the flow path. Additionally, Munters does not allow for cooling fluids other than outside air and cannot be used in applications where recirculation is desired.
U.S. Pat. No. 5,187,946 issued to Rotenberg et al on Feb. 23, 1993, but copies Maisotsenko Russian patent (2096257) discloses a heat exchanger having perforations through the heat exchange plates and alternating wet and dry channels. The present invention is different in substantial ways from the U.S. Pat. No. 5,187,964 disclosure (Maisotsenko Russian Patent 2046257) does not use: a separate treatment of product fluids (cooled fluids, whether gas, as U.S. Pat. No. 5,187,946""s disclosure is limited to, or other fluids), the thin plastic plates that operate as efficient heat transfer from dry channels to wet channels yet do not transfer heat laterally along the surface or the plates, or a shallow slope to the heat exchange plates to allow efficient wicking action, but instead discloses a relatively high angle. It also does not reveal use of a feeder wick, or wick, or reservoir system instead using complex and costly spray heads located in each wet channel. Finally, U.S. Pat. No. 5,187,964 argues against the use of channel guides, urging that turbulent flow provides better efficiency. However, this does not allow U.S. Pat. No. 5,187,964 to control the lateral temperature profile of the individual heat exchange plates. In addition in the current invention by separating the working air stream from the product the working air decreases in flow as it passes through the channel perforations, reducing its pressure drop and at the same time allowing better control of the exhaust channels. U.S. Pat. No. 5,187,964, similar to Munters above is limited to cooling outside air.
U.S. Pat. No. 5,170,633 issued to Kaplan on Dec. 15, 1992, shows the amount of ancillary equipment which can proliferate in indirect evaporative systems. U.S. Pat. Nos., 5,727,394, 5,758,508, 5,860,284, 5,890,372, 6,003,327, 6,018,953, 6,050,100, issued to Belding et al and Goland et al, display the same syndrome of excessive air treatment equipment. In examining systems such as those referred to, it should be borne in mind that a single additional heat exchanger adds more than one third to the overall cost of the system. These systems, aside from the different methods again only apply for cooling air.
U.S. Pat. No. 5,453,223, dated Sep. 26, 1995, and nominally issued to the present applicant, discloses an apparatus in which alternating sets of wet and dry plates provide two streams of air: one dry, cooled by contact with the plates beside it, and one wet, cooled by direct evaporation. However, the unit requires two gas flows in and two gas flows out. In addition, the design in question does not provide for indirect cooling only, without additional direct evaporative cooling. While such a second stage of direct evaporative cooling, raising the humidity of the product air, is often desirable, it is as often not desirable.
Two pending applications by the present inventor also address the technology of indirect evaporative cooling. PCT Application PCT/US01/04082, filed Feb. 7, 2001, discloses one method of eliminating a second stage of direct evaporative cooling. PCT Application PCT/US01/04081 filed on Feb. 7, 2001, discloses better methods of design of the heat exchange cores of indirect evaporative coolers, allowing better wetting and reduced pressure drops.
An indirect evaporative method and apparatus providing more efficient air flow and heat transfer is desirable. The advantages of the improved membrane, reservoir and control mechanism of the indirect evaporative method and apparatus grant advantages over the previously disclosed designs yielding more positive results.
The present invention provides an indirect evaporative cooler of fluids of all types having cross flowing wet and dry channels on opposite sides of a heat exchange plate which allows heat transfer through the plate due to thin plastic construction or other suitable materials but prevents or minimizes heat transfer laterally along the plate. For purposes of application, we wish to define certain terms:
1. Heat transfer surface or heat exchange surface has many configurations. All are encompassed within the subject of this disclosed invention with appropriate adjustment to the wetting and flows as are well known in the industry. For the example of the illustration we make use of a plate configuration.
2. Wet side or portion of the heat exchange surface means that portion having evaporative liquid on or in its surface, thus enabling evaporative cooling of the surface and the absorption of latent heat from the surface.
3. Dry side or portion of the heat exchanger means that portion of the heat exchanger surface where there is no evaporation into the adjacent gas or fluid. Thus, there is no transfer of vapor and latent heat into adjacent gases. In fact, the surface may be wet but not with evaporative fluid or wet by condensation, but no evaporation exists.
4. Working stream or working gas stream is the gas flow that flows along the heat exchange surface on the dry side, passes through the passages in the surface to the wet side and picks up vapor and by evaporation takes latent heat from the heat exchange surface and transports it out into the exhaust. In some embodiments, the working stream may be disposed of as waste and in others it may be used for special purposes, such as adding humidity or scavenging heat.
5. Product stream or product fluid stream is the fluid (gas, liquid or mixture) flow that passes along the heat exchange surface on the dry side and is cooled by the absorption of heat by the working gas stream on the wet side absorbing latent heat by the evaporation in the wet area.
6. Trough wetting system embodiment is a feature of the membrane that on the illustrations occurs in all membranes in a central area of the membranes and the troughs of adjacent membranes work in conjunction as liquid passage ways and as holding locations or reservoirs for purposes of wetting the wet side of the membranes. The location and shape, and relative placement of this trough or trough like features are merely depictions. Other orientation and methods and apparatus are encompassed within the disclosed invention.
The plate also has passageways or perforations or transfer means between the dry side of the plate and the wet side in defined areas providing flow from the dry working channels to the working wet channels in which direct evaporative cooling takes place. By means of the perforations the working gas streams have a pressure drop through the system, which is reduced.
The method of the invention makes use of the separation of a working gas flow (that is used to evaporate liquid in the wet channels and thus to cool the wet surface of the heat exchanger plate) from the product fluid flow, both flowing through dry product channels and dry working channels on the same side of the heat exchange plate and that both give up heat to the heat exchange plate that on its obverse surface is being cooled by evaporation in the working wet channels.
The working gas flow first enters the dry working channel and then through perforations, pores or other suitable means of transfer across the barrier of the plate to the wet side and thence into the wet working channels where evaporation of liquid on the wet channel surface, cools this plate.
The dry product channels are on the dry side of this plate. The plate is of a thin material to allow easy heat transfer across this pate and thus to readily allow heat to transfer from the dry product channel to the wet working channel. This is one basic unit or element of the invention illustrating the method of the separation of working gas flows to indirectly cool the separate product fluid by evaporative cooling.
It is therefore on object of the invention to provide an indirect evaporative cooler having perforations allowing flow from dry working channels to wet working channels on the opposite side of the heat exchange plate.
It is another object of the invention to provide an indirect evaporative cooler having heat exchange plates, which do not allow substantial lateral heat transfer but do allow heat exchange through and across the plate. This produces a temperature transfer across the plate that is not averaged out by lateral heat transfer down the plate. Averaging the temperature down the plate would effectively reduce the temperature difference through and across the plate and results in lower heat transfer rates across the plate. Thus, it is part of this invention to have heat readily transfer across the plate from the dry side to the wet side but not readily transfer along the surface of the plate.
It is another object of the invention to provide an indirect evaporative cooler having a temperature gradient across the two dimensional surface of the plate, and thus providing working gas stream channels having a range of temperatures.
It is another object of the invention to provide an indirect evaporative cooler allowing selection of the product fluid streams for use in cooling, in particular, the fluid streams exiting from the coldest product channels may be selected for use in cooling. Conversely, the selection may be of some portion of the working gas streams to give added humidity to the environment.
It is another object of the invention to provide an indirect evaporative cooler having efficient wicking action allowing easy wetting of substantially all of the surface area of the wet channels with out excess water that cools the water rather then the air.
It is another object of the invention to provide an indirect evaporative cooler having a system providing liquid uniformly to all wet channels of the device. The system has features to quickly distribute the liquid to all wet membranes, to provide reservoirs for wetting and a control system to adjust and control the liquid distribution.
It is yet another object of the invention to provide an indirect evaporative cooler having cycle selection means, so that during summer months, it may be used to provide cooled, non-humidified air, and during winter months, it may be used to scavenge heat from gases exiting a space while simultaneously humidifying the space.
It is yet another object of the invention to provide an efficient indirect evaporative cooler, allowing cooling of a stream of a product to substantially the working gas dew point temperature.
It is another object of the invention to provide an efficient indirect evaporative cooler having a relatively small pressure drop for working gas streams.